This relates to an improved particle counter of the Coulter (R) Counter type and to a method for its manufacture.
U.S. Pat. No. 2,656,508 describes a device for the electronic counting, sizing and analysis of microscopic particles in a fluid suspension. In that device, the fluid suspension is forced to flow through an orifice from one insulated vessel to another. A D.C. electric current is established between the two vessels by mounting electrodes in the two vessels. Since the only electrical path between the two vessels is through the fluid that flows through the orifice, an electric current flow and field are established in the orifice. The orifice and the resultant electric field in and around it constitute a sensing zone. As each particle passes through the sensing zone, the impedance of the contents of the sensing zone will change for the duration of the passage, thereby modulating the current flow and electric field in the sensing zone. The change in current flow and electric field produces a signal that is applied to a detector suitably arranged to respond to such change.
It has been proved that the change in impedance of the contents of the sensing zone as a particle passes through it is approximately proportional to the volume of the particle where the diameter of the orifice is smaller than the axial length of the orifice.
The impedance sensing principle has been extended to provide information concerning particle characteristics such as the composition and nature of the material constituting the particles, as disclosed in U.S. Pat. No. 3,502,974 to Coulter et al. and U.S. Pat. No. 3,502,973 to Coulter et al. These devices generally have at least two current sources, both of which are applied to the sensing zone simultaneously, one having a radio frequency and the other being a "zero frequency" direct current or, alternatively, having a sufficiently low frequency that the reactive part of the particle impedance has a negligible effect on the response of the apparatus. One of the useful particle descriptors that can be obtained from this dual source arrangement is known in the art as the "opacity" of the particles. In a general sense, opacity measures the difference in size as measured at radio frequency as compared to size measured at low or zero frequency.
As is appreciated in the art of cytology, any new particle descriptor that can be measured is useful in identifying, analyzing and sorting particles. For example, cells have a membrane of very high resisitivity which is in the range of a dielectric. However, the internal portion of the cell is fairly conductive, with different types of particles having varying internal resisitivities. Also, it is contemplated that the pathological state of the cell will affect its internal resisitivity. Consequently, it is desirable to measure this internal resisitivity on a cell by cell basis. U.S. Pat. No. 4,298,836, for example, describes apparatus for measuring resisitivity. U.S. Pat. No. 3,710,933 describes an impedance sensing orifice that is used in combination with measurements of light absorbance, scattered light and fluorescent light.
As shown in FIG. 1, illustrative prior art particle counters comprise an inlet chamber 10, an outlet chamber 20, an orifice 30 interconnecting the chambers, viewing optics 40 and electronics 50. A liquid stream of individually suspended particles, originally from a pressurized reservoir (not shown), proceeds through a capillary tube 12 into chamber 10. A laminar liquid sheath, originally from another pressure reservoir (not shown), proceeds through tubes 14 so as to surround the stream of particles. As the liquid stream of particles exits from tube 12, hydrodynamic pressures reduce the diameter of the stream of particles as the stream obtains the velocity of the liquid sheath. The liquid sheath also acts to center the stream of particles so that particles pass through the orifice 30 along a center axis 18, with the elongated particles, if any, having their elongated axis aligned with the center axis 18. After leaving the orifice 30, the particles enter chamber 20. Chamber 20 has an inlet 22 and an outlet 24 for a suitable fluid which removes the suspended particles.
Orifice 30 is a microscopic hole that typically is machined in a pane 32 of jewelry-quality sapphire. The dimensions of the orifice are small enough that a single particle can be counted. To this end the orifice ordinarily has a diameter on the order of four-thousandth of an inch (100 micrometers) or less.
Electronics 50 comprises electrodes 52 and 54 in chambers 10, 20 respectively, a constant current supply 56 and signal processing elements 60. Electrodes 52 and 54 and constant current supply 56 are used to establish an electric current in chambers 10 and 20 which passes through orifice 30 and varies with the passage of particles through the orifice. The signal processing elements illustratively comprise an amplifier 62, a baseline restorer 64, trigger circuitry 66, counter 68, a peak detector 72, and size determining circuitry 74. Details of signal processing circuitry are well known in the art being disclosed, for example, in the above-referenced patents. The signal processing circuitry typically is mounted on a circuit board in a separate package apart from the inlet and outlet chambers.